1. Field of the Invention The present invention relates to an electromagnetic wave absorber that absorbs electromagnetic waves radiated from electronic components such as motor, etc.
2. Description of the Related Art
There have recently seen occasional erroneous operations in electronic components due to electromagnetic disturbances of electronic components such as motor, computer, etc. What is worse, radiation of electromagnetic waves has been known as unfavorable influence upon the human body. To avoid the unfavorable influence of electromagnetic wave radiation, there have been used electromagnetic wave absorbers for absorbing electromagnetic waves. Specifically, electromagnetic wave absorbers have been applied to wall portions surrounding electronic components that radiate electromagnetic waves as well as building walls in order to absorb electromagnetic waves radiated from electronic components.
Incidentally, reduction in film thickness and weight has been required of electromagnetic wave absorbers. For instance, regarding the ETC system (electronic toll collection system) used at toll gates of expressways, employment of electromagnetic wave absorbers have been essential to ceilings and walls of toll gate in order to prevent erroneous operation of an on-vehicle ETC body due to electromagnetic disturbances from adjoining lanes. Therefore, the reduction in film thickness and weight is required of electromagnetic wave absorbers.
As an example of a conventional electromagnetic wave absorber, there has been used a so-called λ/4 type electromagnetic wave absorber of which configuration is designed based on a non-reflecting condition expression that makes electromagnetic waves non-reflective at a surface of the electromagnetic wave absorber.
The λ/4 type electromagnetic wave absorber includes a dielectric layer with a thickness of λ/(4·√∈r) and a relative permittivity of ∈r (λ representing wavelength of electromagnetic wave in a vacuum state, ∈r representing relative permittivity of the dielectric). One surface of the dielectric layer includes an electromagnetic wave reflective layer and the other surface thereof includes an electrical resistance film layer with an electric-wave impedance of 377Ω in the air. In this connection, the dielectric layer of the λ/4 type electromagnetic wave absorber needs at least the thickness of λ/(4·√∈r), which restricts degree of reduction in film thickness.
In recent years, there has been proposed an electromagnetic wave absorber of which dielectric layer thickness is made thinner than λ/(4·√∈r) by inserting a DCF (divided conductive film) between an electrical-resistance film and a reflective layer of a λ/4 type electromagnetic wave absorber (for instance, refer to JP Laid-open Patent Application Publication No. 2004-319788).
However, despite insertion of the DCF (divided conductive film) disclosed in Publication No. 2004-319788, between the electrical resistance film and the reflective layer of the λ/4 type electromagnetic wave absorber, reduction in film thickness and weight for the dielectric layer has not advanced significantly. Further, reduction in film thickness and weight has been required of the DCF as well, in similar with the dielectric layer. However, the reduction in film thickness and weight is not considered for the DCF disclosed in Publication No. 2004-319788.